If Gollum seems a bit off to you when you interact with him, there's a reason why. He has a mental illness. But which one? In 2004, a group of medical students led by Nadia Bashir published an article in the British Medical Journal about their efforts to diagnose the Lord of the Rings character. They examined his behavior and concluded that Gollum has schizoid personality disorder:

Several differential diagnoses need to be considered, and we should exclude organic causes for his symptoms. A space occupying lesion such as a brain tumour is unlikely as his symptoms are long standing. Gollum's diet is extremely limited, consisting only of raw fish. Vitamin B-12 deficiency may cause irritability, delusions, and paranoia. His reduced appetite and loss of hair and weight may be associated with iron deficiency anaemia. He is hypervigilant and does not seem to need much sleep. This, accompanied by his bulging eyes and weight loss, suggests hyperthyroidism. Gollum's dislike of sunlight may be due to the photosensitivity of porphyria. Attacks may be induced by starvation and accompanied by paranoid psychosis.

An internet search found over 1300 sites discussing the nature of Gollum's “mental illness.” We asked 30 randomly selected medical students if they thought Gollum had a mental illness. Schizophrenia was the most common diagnosis (25 students), followed by multiple personality disorder (three). On initial consideration schizophrenia seems a reasonable diagnosis. However, in the context of the culture at the time it is unlikely. Delusions are false, unshakeable beliefs, not in keeping with the patient's culture. In Middle Earth, the power of the ring is a reality. The passivity phenomena Gollum experiences are caused by the ring, and these symptoms occur in all ring bearers. Gollum does not fulfil the ICD-10 criteria for the diagnosis of schizophrenia.3

The presence of two personalities, Gollum and Sméagol, raises the possibility of multiple personality disorder. In this diagnosis one personality is suppressed by the other and the two personalities are always unaware of each other's existence.3 In this case, Gollum and Sméagol occur together, have conversations simultaneously, and are aware of each other's existence.

Gollum displays pervasive maladaptive behaviour that has been present since childhood with a persistent disease course. His odd interests and spiteful behaviour have led to difficulty in forming friendships and have caused distress to others. He fulfils seven of the nine criteria for schizoid personality disorder (ICD F60.1), and, if we must label Gollum's problems, we believe that this is the most likely diagnosis.